Harvesting machines, in particular combines and forage harvesters, are provided with crop recovery attachments whose height can be controlled by means of hydraulic cylinders. Under certain operating conditions it is necessary to lower these crop recovery attachments all the way to the ground and to guide them along the ground with only a low contact force. In order to evade irregularities on the ground, mechanical or pneumatic spring systems are provided in the supply lines to the hydraulic cylinders or between the hydraulic cylinders and the crop recovery attachments that permit the crop recovery attachments to be raised easily and thereby avoid an excessively high ground contact force. In some cases a pressure indicator is used to display the instantaneous contact force to the operator so that manual control of the height can be undertaken. All these possibilities of height control of the attachment on harvesting machines have the disadvantage that they are not sensitive enough and therefore do not respond to small changes in the contact force, or do so only too late.
In addition it is known practice in agricultural tractors (Bosch-Hydraulic, Supplementary Production-Information, K6/VKD 2-BEY005/5 DE (5.82)) to control a ground breaking implement that is coupled to a three-point implement hitch by means of an electronic force sensor that is integrated into a pin. Further developments of force sensors of this type as applied to agricultural tractors are described in a large number of publications. In this connection reference is made, for example, to EP-A1-0 238 875, DE-A1-35 15 126, DE-A1-34 34 131 and U.S. Pat. No. 4,982,613.
The problem underlying the invention is seen as that of proposing a harvesting machine with an attachment whose height is automatically controlled very simply in relation to the contour of the ground with minimal changes in contact force or significant time delay.
According to the invention an electronic output signal can be generated from the resistance of the attachment during its movement across the ground, which is in direct relationship to its contact force, and the signal is processed in a control or regulating device that controls a hydraulic valve which initiates a change in length of the strut and thereby changes the height of the attachment. The use of an electronic control assures an extremely fast response to irregularities of the ground and largely eliminates the influence of friction forces in the strut.
Although the pin with the force sensor could be installed in various ways, for example, in axial alignment with the strut, it is preferred to arrange it in such a way that the forces applied to it act radially so that it can simultaneously act as a pivot bearing.
With very wide attachments it is possible that they are raised by a ground irregularity only at one side or only in the center. In order to assure that any resulting change in contact force does have an effect, several struts and several pins with force sensors are provided. These force sensors are preferably connected to a small computer in such a way that an increase in the applied force at only one pin is sufficient to raise the attachment, while a lowering of the attachment requires reduction of the applied force at all the force sensors. However, not every strut is required to act on a force sensor.
If the various struts are configured differently so that they can absorb as well as apply different levels of force, precise operation of the control is assured if compatibility of the signals generated can be attained, which can be accomplished by the use of differing force sensors or by an amplification or attenuation of the signal in a compensating circuit.
The lowest possible reaction time, that results in an almost constant contact force, is attained when the sensitivity of the force sensor in connection with the control arrangement is as low as 15 millivolts.
An override switch is provided in order to permit the operator to change the height of the attachment manually at the end of a field or under certain conditions.
In order to reduce as far as possible the bearing friction of the struts and the attachment in the bearing arrangement at the frame, needle bearings or journal bearings are provided.
The use of an electronic force sensor of the type HITCHTRONIC TYP KS 5 of Robert Bosch G.m.b.H., in particular, results in a simple installation of an electronic height control of the attachment of a harvesting machine.